Piping dead legs

Instrumentation will include field instruments used in the manufacturing process and other instruments assoeiated with special tasks, such as that of monitoring laboratory or computer room environmental eonditions. The accessibility of instruments must be such as to permit their eleaning and maintenanee. Siting is also important, and instruments should be installed as close to the point of measurement as possible. The placement of flowmeters in piping dead-legs should be avoided. Careful consideration should also be given to the appropriate position of other instruments sueh as thermometers and thermocouples so that they, too, can fulfill their measurement and eontrol functions. Construction materials that come into direct contact with the pharmaceutical or healthcare production process stream must not contaminate or affect the manufactured product in any way. Instrument lubricants and coolants must not come into contact with in-process product or equipment. The reliability of instruments should also be considered for instance, a pressure transmitter that uses atmospheric pressure as its reference may suffer from poor reliability. A draft calibration schedule may also be prepared. 

Poor performance can be caused by inadequate winterization of pipes, dead legs, and instruments. There have been cases of equipment damage, and even explosions, initiated by freezing of water or process fluids in dead legs of a distillation system. Careful design, inspection, and testing is required in cold climates. Detailed discussion is available elsewhere (120, 266). 

Chemical cleaning was considered since small-diameter pipe had been plugged in dead-leg regions. At the time of writing, no decision had been made concerning cleaning. 

One method of operating a non-chlorinated supply safely is to ensure that the line runs constantly to waste, as do very old drinking fountains. This avoids biological growth that can accumulate in stagnant water. Dead legs in the piping system are always undesirable, especially in such cases. 

As mentioned in Chapter 3, DI water has now replaced distilled water for most laboratory purposes. PVC pipe, which is both inexpensive and easy to install, is normally employed to carry the water to points of usage. It is very important that the lines do not have dead legs where water is allowed to stand, since this could encourage bacterial growth. In cases where sterile water is needed, special steps must be taken. The water may be boiled, which also drives out dissolved gases, or ultraviolet radiation may be employed, a common method in the cosmetics industry. Membrane filters will also effectively remove bacteria. Laboratories needing sterile water often use two bacteria removal systems of... 

Linden, NJ, USA, Refinery, Explosion/Fire Failure of a dead leg piping released vapors 17,500,000... 

No dead leg >6 of the internal diameter of the unused pipe (measured from the axis of the pipe in use)... 

Temporary circulation pumps are often necessary, as permanently installed pumps may be unsuitable or unable to provide the velocity necessary for flushing and cleaning work. Typically the water velocity necessary is 6 to 9 ft/sec (2 to 3 m/sec). Once in motion, debris velocity is virtually independent of particle size, but is clearly a function of water velocity. Relatively high velocities are needed to lift particles up from horizontal pipe surfaces and dirt pockets (dirt collection points at the base of riser pipes), and especially to overcome problems of particle retardation when moving from the horizontal through a vertically upward bend. Special provisions may need to be made to ensure the cleaning of dead-leg areas,... 

In-line instruments or sensors are necessary components for automated processes. For ease of cleaning, sensors should be chosen that directly mount onto vessel nozzles or piping tees with minimum dead leg distances. Also, the instru-... 

Piping is sloped to provide for complete drainage of the system. The system is free of dead legs. 

The discharge pipe from a storage vessel inevitably constitutes a dead-leg and provision should be made for back-flushing with water to remove any blockages. 

In piping design, the three conditions that lead to corrosion problems are (i) water traps (ii) dead legs (iii) high velocities. 

Product contamination leaking valves/crud left in storage tanks/crud left in dead legs in piping/corrosion products/unexpected chemical reactions/sampling fault/ analysis fault/unexpected solubility effects. 

System design - Extended piping runs, dead-legs and disinfected holding tanks. 

Identify Piping System Dead legs and Eliminate if Possible ... 

Another potential problem to watch for concerns safety shower systems that use a Christmas tree arrangement of piping. If someone trips the flow to one shower, the entire system will fill up with water, hence the lines to those showers that were not tripped will have a dead-leg of water that can quickly freeze— leading to line rupture when the water is thawed. 

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